Abstract

This CSS module defines pseudo-elements, abstract elements that represent portions of the CSS render tree that can be selected and styled.

CSS is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, in speech, etc.

Status of this document

This section describes the status of this document at the time of
its publication. Other documents may supersede this document. A list of
current W3C publications and the latest revision of this technical report
can be found in the W3C technical reports
index at http://www.w3.org/TR/.

This document is a First Public Working Draft.

Publication as a First Public Working Draft does not imply endorsement by the W3C
Membership. This is a draft document and may be updated, replaced or
obsoleted by other documents at any time. It is inappropriate to cite this
document as other than work in progress.

The (archived) public
mailing list www-style@w3.org (see
instructions) is preferred
for discussion of this specification. When sending e-mail, please put the
text “css-pseudo” in the subject, preferably like this:
“[css-pseudo] …summary of comment…”

1. Introduction

This section is informative.

Pseudo-elements represent abstract elements of the document
beyond those elements explicitly created by the document language.
Since they are not restricted to fitting into the document tree,
the can be used the select and style portions of the document
that do not necessarily map to the document’s tree structure.
For instance, the ::first-line pseudo-element can
select content on the first formatted line of an element
after text wrapping,
allowing just that line to be styled differently
from the rest of the paragraph.

Each pseudo-element is associated with an originating element
and has syntax of the form ::name-of-pseudo.
This module defines the pseudo-elements that exist in CSS
and how they can be styled.
For more information on pseudo-elements in general,
and on their syntax and interaction with other selectors,
see [SELECTORS4].

2.
Typographic Pseudo-elements

2.1.
The ::first-line pseudo-element

The ::first-line pseudo-element describes the contents of
the first formatted line of its originating element.

The rule below means
“change the letters of the first line of every p element to uppercase”:

p::first-line { text-transform: uppercase }

The selector p::first-line
does not match any real document element.
It does match a pseudo-element that conforming user agents
will insert at the beginning of every p element.

Note: Note that the length of the first line depends on a number of factors,
including the width of the page, the font size, etc.

<P>This is a somewhat long HTML
paragraph that will be broken into several
lines. The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.</P>

The lines might be broken as follows:

THIS IS A SOMEWHAT LONG HTML PARAGRAPH THAT
will be broken into several lines. The first
line will be identified by a fictional tag
sequence. The other lines will be treated as
ordinary lines in the paragraph.

This paragraph might be “rewritten” by user agents
to include a fictional tag sequence to represent ::first-line.
This fictional tag sequence helps to show how properties are inherited.

<P><P::first-line> This is a somewhat long HTML
paragraph that </P::first-line> will be broken into several
lines. The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.</P>

If a pseudo-element breaks up a real element,
the desired effect can often be described by a fictional tag sequence
that closes and then re-opens the element.

Thus, if we mark up the previous paragraph with a span
element encompassing the first sentence:

<P><SPAN class="test"> This is a somewhat long HTML
paragraph that will be broken into several
lines.</SPAN> The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.</P>

the user agent could simulate start and end tags for span
when inserting the fictional tag sequence for ::first-line
to get the correct inheritance behavior.

<P><P::first-line><SPAN class="test"> This is a somewhat long HTML
paragraph that will </SPAN></P::first-line><SPAN class="test"> be broken into several
lines.</SPAN> The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.</P>

2.1.1.
Finding the First Formatted Line

In CSS, the ::first-line pseudo-element
can only have an effect when attached to a block container.
The first formatted line of an element
must occur inside a block-level descendant in the same flow
(i.e., a block-level descendant that is not out-of-flow due to floating or positioning).

For example, the first line of the DIV in <DIV><P>This line...</P></DIV>
is the first line of the P
(assuming that both P and DIV are blocks).

The first line of a table-cell or inline-block
cannot be the first formatted line of an ancestor element.
Thus, in <DIV><P STYLE="display: inline-block">Hello<BR>Goodbye</P> etcetera</DIV>
the first formatted line of the DIV is not the line "Hello".

Note: Note that the first line of the p in this fragment:
<p><br>First...
doesn’t contain any letters (assuming the default style for br).
The word "First" is not on the first formatted line.

A user agent must act as if the fictional start tags of a ::first-line pseudo-element
were nested just inside the innermost enclosing block-level element.

During CSS inheritance, the portion of a child element that occurs on the first line
only inherits properties applicable to the ::first-line pseudo-element
from the ::first-line pseudo-element.
For all other properties inheritence is
from the non-pseudo-element parent of the first line pseudo element.
(The portion of a child element that does not occur on the first line
always inherits from the parent of that child.)

2.2.
The ::first-letter pseudo-element

The ::first-letter pseudo-element represents
the first typographic letter unit[CSS3TEXT]
on the first formatted line of its originating element,
if it is not preceded by any other content
(such as images or inline tables) on its line.
The ::first-letter pseudo-element can be used
to create “initial caps” and “drop caps”,
which are common typographic effects.

For example, the following rule creates a 2-line drop-letter
on every paragraph following a level-2 header,
using the initial-letter property defined in [CSS3LINE]:

h2 + p::first-letter { initial-letter: 2; }

Punctuation (i.e, characters that belong to the Punctuation (P*) Unicode general category[UAX44])
that precedes or follows the first typographic letter unit must also be included
in the ::first-letter pseudo-element.

As explained in [CSS3TEXT],
a typographic letter unit can include more than one Unicode codepoint.
For example, combining characters must be kept with their base character.
Also, languages may have additional rules
about how to treat certain letter combinations.
In Dutch, for example, if the letter combination "ij" appears at the beginning of an element,
both letters should be considered within the ::first-letter pseudo-element. [UAX29]
The UA should tailor its definition of typographic letter unit
to reflect the first-letter traditions of the originating element’s content language.

This is actually a problem in cases where the originating element is an ancestor
with a different content. What should we say here?

Note: Note that the first typographic letter unit may in fact
be a digit, e.g., the “6” in “67 million dollars is a lot of money.”

If the characters that would form the ::first-letter
are not in the same element, such as "‘T" in <p>'<em>T...,
the user agent may create a ::first-letter pseudo-element
from one of the elements, both elements, or simply not create a pseudo-element.
Additionally, if the first letter(s) of the block
are not at the start of the line (for example due to bidirectional reordering),
then the user agent need not create the pseudo-element(s).

The ::first-letter pseudo-element is contained within any ::first-line
pseudo-elements, and thus inherits from ::first-line.

2.2.1.
Finding the First Letter

The first letter must occur on the first formatted line.
For example, in this HTML fragment: <p><br>First...
the first line doesn’t contain any letters
and ::first-letter doesn’t match anything.
In particular, it does not match the “F” of “First”.

In CSS, the ::first-letter pseudo-element
only applies to block containers.
A future version of this specification
may allow this pseudo-element to apply to more display types.
The ::first-letter pseudo-element can be used
with all such elements that contain text,
or that have a descendant in the same flow that contains text.
A user agent should act as if the fictional start tag
of the ::first-letter pseudo-element
is just before the first text of the element,
even if that first text is in a descendant.

In CSS the first letter of a table-cell or inline-block
cannot be the first letter of an ancestor element.
Thus, in <DIV><P STYLE="display: inline-block">Hello<BR>Goodbye</P> etcetera</DIV>
the first letter of the DIV is not the letter "H".
In fact, the DIV doesn’t have a first letter.
If an element is a list item (display: list-item),
the ::first-letter applies
to the first letter in the principal box after the marker.
User-Agents may ignore ::first-letter
on list items with list-style-position: inside.
If an element has ::before or ::after content,
the ::first-letter applies to the first letter of the
element including that content.

Example:
After the rule p::before {content: "Note: "}, the
selector p::first-letter matches the "N" of "Note".

2.2.2.
Styling the ::first-letter Pseudo-element

In CSS a ::first-letter pseudo-element is similar to an inline-level element
if its float property is none;
otherwise, it is similar to a floated element.
The following properties that apply to ::first-letter pseudo-elements:

any other properties defined to apply to ::first-letter
by their respective specifications

User agents may apply other properties as well.

Note: In previous levels of CSS,
User Agents were allowed to choose a line height, width and height based on the shape of the letter,
approximate font sizes,
or to take the glyph outline into account when formatting.
This possibility has been intentionally removed,
as it proved to be a poor solution for the intended use case (Drop Caps),
yet caused interoperability problems.

Example:
This CSS and HTML example shows a possible rendering of an initial cap.
Note that the fictional start tag of the first letter
is inside the span,
and thus the font weight of the first letter is normal,
not bold as the span:

<P>
<SPAN>
<P::first-letter>
T
</P::first-letter>he first
</SPAN>
few words of an article in the Economist.
</P>

Note that the ::first-letter pseudo-element tags abut the
content (i.e., the initial character), while the ::first-line
pseudo-element start tag is inserted right after the start tag of the
block element.

3.2.
Styling Highlights

Are there any other properties that should be included here?
[Caret-color should be added once we have it.]

Note: Historically (and at the time of writing)
only color and background-color have been interoperably supported.

If color is not specified, the text (and text decoration)'s
unselected color must be used for the highlight.
(As usual, the initial background-color is transparent.)
Issue: Can we reuse currentColor for this, now that it computes to itself?

The UA should use the OS-default highlight colors
when neither color nor background-color has been specified by the author.

Note: This paired-cascading behavior
does not allow using the normal cascade
to represent the OS default selection colors.
However it has been interoperably implemented in browsers
and is thus probably a Web-compatibility requirement.

The color property specifies the color of both the text
and all line decorations (underline, overline, line-through)
applied to the text.
Note: Because text-emphasis-color defaults to currrentColor,
it will by default also set the color of the the emphasis marks.

3.3.
Area of a Highlight

For each instance of highlighting
there exists a root highlight overlay
which is a tesselated overlay
over the entire tree,
the actively-highlighted portions of which are represented
by the corresponding highlight pseudo-element.
The ::selection overlay is drawn
over the ::spelling-error overlay
which is drawn over the ::grammar-error overlay.

Each box owns a piece of the overlay corresponding to any text or replaced content
directly contained by the box.

For text, the corresponding overlay must cover at least the entire em box
and may extend further above/below the em box to the line box edges.
Spacing between two characters may also be part of the overlay,
in which case it belongs to the innermost element that contains both characters
and is selected when both characters are selected.

For replaced content, the associated overlay must cover at least the entire replaced object,
and may extend outward to include the element’s entire content box.

The overlay may also include other other areas within the border-box of an element;
in this case, those areas belong to the innermost such element that contains the area.

Not sure if this is the correct way of describing the way things work.

3.4.
Cascading and Per-Element Highlight Styles

Each element draws its own portion of the root highlight overlay,
which receives the styles specified by the highlight pseudo-element styles
for which it or one of its ancestors is the originating element.
When multiple styles conflict,
the winning style is the one belonging to the innermost element
after cascading.

The highlight would be green throughout,
with a yellow text outside the <em> element
and a orange text inside it.

This could alternately be described in terms of inheritance.
The observable differences would be in how inherit and unset behave.
Should it inherit from the parent ::selection
or the originating element?
Opera does the former, Gecko/Blink the latter.

Authors wanting multiple selections styles should use
:root::selection
for their document-wide selection style,
since this will allow clean overriding in descendants.
(::selection alone would apply to every element in the tree,
overriding the more specific styles of any ancestors.)

3.5.
Painting the Highlight

The ::selection pseudo element draws its background
over the selected portion of the element,
immediately below any positioned descendants
(i.e. just before step 8 in CSS2.1§E.2).
It also suppresses the drawing of any selected text
(and any text decorations applied to that text)
and instead redraws that text
(and its decorations)
over that background
using the specified color.

What should happen with text shadows?
Drawing them in their original color is disconcerting if that color is not a shade of gray.

For non-replaced content, the UA must honor the color and background-color
(including their alpha channels) as specified.
However, for replaced content, the UA should create a semi-transparent wash
to coat the content so that it can show through the selection.
This wash should be of the specified background-color if that is not transparent,
else the specified color;
however the UA may adjust the alpha channel if it is opaque.

3.6.
Security Considerations

Because the styling of spelling and grammar errors
can leak information about the contents of a user’s dictionary
(which can include the user’s name and even includes the contents of his/her address book!)
UAs that implement ::spelling-error and ::grammar-error
must prevent pages from being able to read
the styling of such highlighted segments.

4.
Tree-Abiding Pseudo-elements

When their computed content value is not none,
these pseudo-elements generate boxes
as if they were immediate children of their originating element,
and can be styled exactly like any normal document-sourced element in the document tree.
They inherit any inheritable properties from their originating element;
non-inheritable properties take their initial values as usual.
[CSS3CASCADE]

For example, the following rule inserts the string “Note: ”
before the content of every <p> element
whose class attribute has the value note:

p.note::before { content: "Note: " }

Since the initial value of display is inline,
this will generate an inline box.
Like other inline children of <p>,
it will participate in <p>’s inline formatting context,
potentially sharing a line with other content.

As with the content of regular elements,
the generated content of ::before and :after pseudo-elements
may be included in any ::first-line and ::first-letter pseudo-elements
applied to its originating element.

For compatibility with existing style sheets written against CSS Level 2 [CSS21],
user agents must also accept the previous one-colon notation
(:before and :after)
for these pseudo-elements.

The ::placeholder pseudo-element represents
placeholder text in an input field:
text that represents the input
and provides a hint to the user on how to fill out the form.
For example, a date-input field
might have the placeholder text “YYYY/MM/DD”
to clarify that numeric dates are to be entered in year-month-day order.

In interactive media, placeholder text is often hidden once the user has entered input;
however this is not a requirement, and both the input value and the placeholder text may be visible simultaneously.
The exact behavior is UA-defined.
Note that in static media (such as print)
placeholder text will be present even after the user has entered input.

Current bikeshedding, based on implementations and discussions,
appears to select from the list
::placeholder,
::placeholder-text,
::input-placeholder.
The WG is going with ::placeholder.
If there are other opinions that provide some kind of argument to the contrary,
they should go to www-style@w3.org.

6.
Additions to the CSS Object Model

Pseudo-elements should be reachable by script,
stylable from script,
and available as event targets.

Note We may extend this
section in the future to allow creation of pseudo-elements from script.

This entire section is a starting point for discussion.
Feedback is welcome.
Implementations, at this point, are not,
as we are not yet sure of our approach.

6.1.
Interface CSSPseudoElement

The CSSPseudoElement interface
allows pseudo-elements to be styleable from script
and makes them event targets.

The approach in this draft
is to start with a bare minimum
for the CSSPseudoElement interface
and build up from there.
Another more radical approach
could take everything that’s common
between a pseudo-element and a node
and create a new base class
for both Node and CSSPseudoElement.

The type attribute
is a string representing the type of the pseudo-element.
This can be one of the following values:

‘before’

The pseudo-element was created before the element’s contents.

‘after’

The pseudo-element was created after the element’s contents.

‘letter’

The pseudo-element is the first letter of the element.

‘line’

The pseudo-element is the first line of the element.

‘selection’

The selection pseudo-element for the element.

The style attribute
is a CSSStyleDeclaration[CSSOM]
that allows directly setting style information (inline styles) onto the pseudo-element.
Inline styles on a CSSPseudoElement have precedence over all
style rules styling that pseudo-element.

This should cascade like actual inline styles, not be a different thing.

The length attribute
represents the number of CSSPseudoElement in the
collection or zero if it is empty.
The method item()
is used to retrieve a CSSPseudoElement by index.
It takes one parameter being the requested index into the collection.
Its return value is the CSSPseudoElement
at the requested index in the collection
or null if that is not a valid index.

The method getByType()
is used to retrieve a CSSPseudoElement
by its type.
Its return value is the CSSPseudoElement
in the collection that matches the type
or null if there is no CSSPseudoElement
in the collection for that type.

6.3.
Addition to the window interface

A new method is added to the Window interface to retrieve
pseudo-elements created by a given element for a given type:

The getPseudoElements() method
is used to retrieve all CSSPseudoElement instances
created by the element elt for the type type.
Its return value is a CSSPseudoElementList,
potentially empty if no pseudo-element exists for the given element and the given type.

Acknowledgements

The editors would like to thank the following individuals for their
contributions, either during the conception of the specification or during
its development and specification review process:
Tab Atkins,
David Baron,
Razvan Caliman,
Chris Coyier,
Anders Grimsrud,
Vincent Hardy.

Conformance

Document conventions

Conformance requirements are expressed with a combination of
descriptive assertions and RFC 2119 terminology. The key words "MUST",
"MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this
document are to be interpreted as described in RFC 2119.
However, for readability, these words do not appear in all uppercase
letters in this specification.

All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words "for example"
or are set apart from the normative text with class="example",
like this:

This is an example of an informative example.

Informative notes begin with the word "Note" and are set apart from the
normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are
set apart from other normative text with <strong class="advisement">, like
this:
UAs MUST provide an accessible alternative.

Conformance classes

Conformance to this specification
is defined for three conformance classes:

A style sheet is conformant to this specification
if all of its statements that use syntax defined in this module are valid
according to the generic CSS grammar and the individual grammars of each
feature defined in this module.

A renderer is conformant to this specification
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the features defined
by this specification by parsing them correctly
and rendering the document accordingly. However, the inability of a
UA to correctly render a document due to limitations of the device
does not make the UA non-conformant. (For example, a UA is not
required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification
if it writes style sheets that are syntactically correct according to the
generic CSS grammar and the individual grammars of each feature in
this module, and meet all other conformance requirements of style sheets
as described in this module.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to
assign fallback values, CSS renderers must
treat as invalid (and ignore
as appropriate) any at-rules, properties, property values, keywords,
and other syntactic constructs for which they have no usable level of
support. In particular, user agents must not selectively
ignore unsupported component values and honor supported values in a single
multi-value property declaration: if any value is considered invalid
(as unsupported values must be), CSS requires that the entire declaration
be ignored.

Experimental implementations

To avoid clashes with future CSS features, the CSS2.1 specification
reserves a prefixed
syntax for proprietary and experimental extensions to CSS.

Prior to a specification reaching the Candidate Recommendation stage
in the W3C process, all implementations of a CSS feature are considered
experimental. The CSS Working Group recommends that implementations
use a vendor-prefixed syntax for such features, including those in
W3C Working Drafts. This avoids incompatibilities with future changes
in the draft.

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage,
non-experimental implementations are possible, and implementors should
release an unprefixed implementation of any CR-level feature they
can demonstrate to be correctly implemented according to spec.

To establish and maintain the interoperability of CSS across
implementations, the CSS Working Group requests that non-experimental
CSS renderers submit an implementation report (and, if necessary, the
testcases used for that implementation report) to the W3C before
releasing an unprefixed implementation of any CSS features. Testcases
submitted to W3C are subject to review and correction by the CSS
Working Group.

Not sure if this is the correct way of describing the way things work. ↵

This could alternately be described in terms of inheritance.
The observable differences would be in how inherit and unset behave.
Should it inherit from the parent ::selection
or the originating element?
Opera does the former, Gecko/Blink the latter. ↵

What should happen with text shadows?
Drawing them in their original color is disconcerting if that color is not a shade of gray. ↵

Current bikeshedding, based on implementations and discussions,
appears to select from the list
::placeholder,
::placeholder-text,
::input-placeholder.
The WG is going with ::placeholder.
If there are other opinions that provide some kind of argument to the contrary,
they should go to www-style@w3.org. ↵

This entire section is a starting point for discussion.
Feedback is welcome.
Implementations, at this point, are not,
as we are not yet sure of our approach. ↵

The approach in this draft
is to start with a bare minimum
for the CSSPseudoElement interface
and build up from there.
Another more radical approach
could take everything that’s common
between a pseudo-element and a node
and create a new base class
for both Node and CSSPseudoElement.
↵

This should cascade like actual inline styles, not be a different thing. ↵